linux-kernel - Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

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Message-ID: <20071211084059.3d03e11d@tux.DEF.witbe.net>
Date:	Tue, 11 Dec 2007 08:40:59 +0100
From:	Paul Rolland <rol@...917.net>
To:	David Newall <david@...idnewall.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Krzysztof Halasa <khc@...waw.pl>,
	Rene Herman <rene.herman@...access.nl>,
	Pavel Machek <pavel@....cz>, Andi Kleen <andi@...stfloor.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"David P. Reed" <dpreed@...d.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, rol@...be.net
Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64
 with MCP51 laptops

Hi,

On Tue, 11 Dec 2007 12:12:59 +1030
David Newall <david@...idnewall.com> wrote:

> H. Peter Anvin wrote:
> > David Newall wrote:
> >
> > I think a single ISA bus transaction is 1 µs, so two of them back to 
> > back should be 2 µs, not 8 µs...
> 
> Exactly.  You think it's 2us, but the documentation doesn't say.  The _p 
> functions are generic inasmuch as they provide an unspecified delay.  

Well, if the delay is so much unspecified, what about _reading_ port 0x80 ?
Will the delay be shorter ? And if so, what about reading port 0x80 and
writing the value back ?
inb  al,0x80
outb 0x80,al

I've been wondering since the beginning of this thread if the problem is not
just the value we put to port 0x80, not writing to the port...

Just my 0.02 Eur...

Paul


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